The mammalian target of rapamycin complex 1 (mTORC1) is an evolutionarily conserved multi- protein kinase complex that couples nutrient and growth factor sensing to regulate a wide array of essential cellular processes. mTORC1 stimulates protein synthesis and ribosome biogenesis, which are vital cellular processes that regulate cell growth and proliferation. Hyperactivation of mTORC1 signaling is found in many types of cancer cells and leads to aberrant increases in protein synthesis and uncontrolled cell proliferation. Despite the positive roles of mTORC1 in enhancing general cap- dependent translation through eIF4E activation, recent studies reveal mTORC1 preferentially stimulates a subset of mRNAs, including those that encode proteins required for translation, and cell growth, proliferation, and invasion. Interestingly, these mRNAs possess pyrimidine-enriched sequences (PES) such as 5'TOP in their 5'UTRs that are thought to play important roles in their expression. However, the molecular mechanisms by which mTORC1 primarily facilitates the translation of a subset of those mRNAs remain elusive. We have discovered a novel mTORC1 substrate, LARP1 that directly interacts with the PES in the 5'UTRs of 5' TOP mRNAs. Our preliminary data suggest that LARP1 functions not only as a molecular switch to regulate PES- containing mRNA translation in response to mTOR activity, but also as a regulator for mTORC1 to phosphorylate 4EBP1 and S6K1 on translationally-competent mRNAs. Knockdown of LARP1 significantly reduces translation of mRNAs that interact with LARP1 and dramatically inhibits ribosome biogenesis and cell proliferation in various cancer cells. We propose that LARP1 is an atypical mTORC1 substrate that regulates the translation of specific PES mRNAs and enhances their mTORC1-dependent translation. We will explore the molecular mechanisms by which mTORC1- dependent phosphorylation of LARP1 contributes to the translation of its interacting mRNAs. Second, we will determine the functional importance of the 5'UTR PES motif enriched in LARP1-bound mRNAs. Finally, we will explore the in vivo functions of LARP1 as well as pathophysiological role of LARP1 in PI3K-Akt-mTORC1-mediated cancer development in animal models. We will accomplish our goals by exploiting a multifaceted but integrated approach combining biochemistry, molecular biology, proteomics, high-throughput RNA sequencing, and mouse models. By elucidating the molecular mechanisms of mTORC1-LARP1-depenent translation, our proposed studies will provide new insights into the regulations of mTORC1-mediated translation and lead to new therapeutic approaches for cancer that target downstream effectors of mTORC1 activity.